Evolution of shale wetting properties under long-term CO 2 /brine/shale interaction: Implications for CO 2 storage in shale reservoirs

In the case of Carbon Geological Storage (CGS), the wettability is one of the main parameters controlling the CO 2 injection capacity and CO 2 plume movement, which affects the safety of long-term CO 2 storage. A full evaluation of this parameter is essential for risk evaluation and storage capacity estimation. The objective of this study was to investigate the wettability changes of shales from the Chang 7 Formation in the Ordos Basin by exposing the samples to ScCO 2 /brine for 10/20/30/and 60 days. The mineral composition, pore structure, chemical functional groups, and surface morphology changes of the shale were analyzed using x-ray diffractometer (XRD), lowtemperature nitrogen adsorption (LN-N 2 ), infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). In addition, the solid -drop method was used to measure the contact angle of shale samples at different times of ScCO 2 soaking. The results show that after 60 days of continuous ScCO 2 soaking, the shale water contact angle and chemical group peak spectrum generally show an increasing trend, which is caused by the reduction of carbonate and clay mineral content. The prolonged time of ScCO 2 soaking weakens the shale ' s water wettability and reduces the water-locking effect (water has a sealing effect in the pore channel of the reservoir), capillary force, and the height of CO 2 sequestration columns, which is favorable for shale gas extraction (a method of extracting natural gas from shale rock by CO 2 fracturing), but it may increase the CO 2 geological leakage risk. The results have important implications for regulating the change of shale water wettability, and this study can also better provide theoretical references for Carbon Geological Storage programs.